It has long been known that distributed schemes for production, storage, and retrieval of electric energy can make electric power distribution by public utilities more cost effective. For example, small or medium-scale facilities storing energy in the form of charged batteries, pumped water, compressed gas, or the like can be connected to the electric power grid and used to add power to the grid during outages and periods of high demand. Conversely, the distributed storage facilities can draw energy from the grid during periods of excess production or low demand, and store the energy for future use. Economic incentives can be provided to the owners of the storage facilities. This can be advantageous to the utilities through a reduction in the fluctuation of demand for electric power. Thus, it can help them to meet their commitments for the delivery of electric power without making capital investments in excess production capacity. The term “load balancing” is often used to refer to schemes of this kind.
One example of a small-scale energy storage facility is the battery system in an electric drive vehicle. Such a battery system may be charged exclusively from the electric power grid, or it may be charged through on-board electrical generation (consuming liquid or gaseous fuel), or it may be charged through some combination of the two preceding methods. Although the electric-drive vehicles that have received the most attention recently are automobiles (“electric cars”), other electric drive vehicles of interest in this context include trucks, and possibly also boats and trains.
In so-called “vehicle-to-grid (V2G)” schemes, the batteries within electric cars (or other electric drive vehicles) are connected, at times, to the power grid and used for load balancing. Such schemes have been under discussion at least since the publication in 1997 of the paper by W. Kempton and S. E. Letendre, “Electric Vehicles as a New Power Source for Electric Utilities.” Transpn. Res.-D, vol. 2, no. 3, (1997), pp. 157-175.
The premise of V2G is that the battery within an electric car represents an unused resource when the car is not being used. When deployed to a large portion of the population of a city, this unused resource has a significant electrical power storage capability. Therefore, the electrical power utility could charge or discharge the batteries of electric cars which are plugged into the power grid, e.g. while the owners are at work, shopping, or at home. The charging or discharging would be timed to reduce fluctuations in supply and demand, with the possible beneficial consequences that primary production capacity is used more efficiently and overall electrical consumption is reduced.
One drawback of V2G as currently envisaged is that it may have limited acceptability to consumers. That is, a typical consumer will consider it desirable to always maintain the battery in a full (i.e., fully charged) condition, whereas the utility company will want to alternately charge and discharge the battery according to fluctuations in supply and demand. This poses a problem not only because the consumer will view the utility company's use as denying him the full benefit of his property (i.e., the battery), but also because every car battery has a limited number of charge/discharge cycles, and the utility company's use will therefore reduce the lifetime of the battery.
Of course, the utility company may provide incentives to the consumer in the form of reduced electricity prices, or ownership of the batteries within the car may be transferred to the utility. Although this may overcome some resistance by consumers, the basic conflict between maintaining a full charge and permitting charge flexibility is left unresolved.